1. Field of the Invention
The present invention relates to an electric double-layer capacitor using a certain carbon material and, more particularly, to a carbon acting as an active material used in polarized electrodes that are employed in an electric double-layer capacitor. The invention relates also to a method of fabricating the aforementioned carbon, a method of selecting the constituent materials of an electrolyte solution for use in an electric double-layer capacitor, a method of fabricating such an electric double-layer capacitor, and its usage.
2. Description of the Related Art
Electric double-layer capacitors (also known as supercapacitors, ultracapacitors, pseudocapacitors, hybrid capacitors, or electrochemical capacitors) capable of being charged and discharged with large currents are promising as electric power storage devices that are frequently charged and discharged such as batteries for electric vehicles, auxiliary power supplies for solar batteries, and auxiliary power supplies for wind generators. Therefore, there is a demand for an electric double-layer capacitor having a high energy density, being capable of being quickly charged and discharged, and having excellent durability (see, for example, xe2x80x9cFourth Symposium On State-Of-The-Art Techniques Regarding EV/HEV: Present Situation Of Capacitor Technologies And Problems To Be Solvedxe2x80x9d, Executive Committee On xe2x80x9cInternational Symposium On State-Of-The-Art Techniques Regarding Batteries For Electric Vehiclesxe2x80x9d, Nov. 8, 1999).
In an electric double-layer capacitor, a pair of polarized electrodes are located opposite to each other via a separator within an electrolyte solution to form positive and negative electrodes. In principle, electric charge is stored in an electric double-layer formed at the interface between each polarized electrode and the electrolyte solution. Accordingly, only activated carbon having a large specific surface area has been used in the past because it has been considered that the capacitance of the electric double-layer capacitor is roughly proportional to the surface area of the polarized electrodes.
In other words, a substance having a maximum surface area per unit weight has been selected as the material of the polarized electrodes because the electric double-layer capacitor is formed at the interface between a conductive material in solid phase and an electrolyte solution. In practice, the aforementioned xe2x80x9cper unit weightxe2x80x9d should be read as xe2x80x9cper unit volumexe2x80x9d, since the space consisting of thin holes between the carbon particles forming the electrode and inside the carbon particles is filled with the electrolyte solution and the weight of the electrolyte solution is added.
Such activated carbon is fabricated by carbonizing a carbonaceous material at a temperature lower than 500xc2x0 C. and then activating the material. The activating operation is carried out, for example, by heating the material to 600xc2x0 C. to 1000xc2x0 C. within an atmosphere of water vapor, carbon dioxide, or the like or mixing zinc chloride, potassium hydroxide, or the like into the material and heating the mixture within an inert atmosphere. Micropores are created by the activating process, thus increasing the specific surface area of the activated carbon. Specific surface areas measured by a BET gas absorption measurement method are generally about 1,000 to 2,000 m2/g.
On the other hand, it has already been proposed to provide a novel polarized electrode made of a carbon having a specific surface area of about 300 to 400 m2/g and an interplanar spacing (inter-layer distance) (d002) of 0.365 to 0.385 nm in order to obtain an electric double-layer capacitor having a large capacitance (Japanese patent laid-open No. 11-317333). In particular, an easily graphitizable carbon containing a large amount of crystallites of a multilayer graphite structure having a well developed carbon mesh surface structure is prepared. This carbon is dry distilled at 700 C. to 850 C. to remove the volatile components. The remaining material is thermally treated together with KOH at 800 C. Using this carbon, an improvement of about 40% in capacitance over the electric double-layer capacitor using the prior art activated carbon has been accomplished.
However, the electric double-layer capacitor proposed by the present applicant as mentioned above suffers from some difficulties. That is, the capacitance decreases during repeated use. This involves generation of gas. Also, the internal resistance increases. In addition, the mechanism of capacitance production is not understood.
Accordingly, it is an object of the present invention to provide an electric double-layer capacitor which is free of the foregoing problems and has improved energy density and power density.
We have considered the capacitance generation mechanism of the above-described carbon fabricated by the aforementioned method from an easily graphitizable carbon in which crystallites of the multilayer graphite structure described above have developed. That is, a large capacitance is created despite the fact that the surface area is small. We have analyzed the cause of generation of gas when the capacitor is operated at a voltage of 3.0 V or higher and the cause of deterioration of the characteristics.
Thus, it is a more specific object of the invention to provide an electric double-layer capacitor which has an improved capacitance per unit volume, improved repeated use durability at an operating voltage of 3.0 V or higher, a decreased internal resistance, and an enhanced power density, and which can be quickly charged and discharged.
It is important to improve the stability of the capacitance value and the usable voltage when improving the energy density. It is important that the response speed is improved and the internal resistance is reduced when enhancing the power density.
In summary, the present invention provides a novel electric double-layer capacitor which uses a carbon material entirely different in characteristics from those of the prior art activated carbon and which produces a capacitance by a mechanism entirely different from the capacitance generation mechanism of an electric double-layer capacitor using the prior art activated carbon.
We have found that an improved capacitance is obtained by preparing an easily graphitizable carbon in which crystallites of multilayer graphite have developed, dry distilling the carbon at 700xc2x0 C. to 850xc2x0 C., treating the resulting carbon together with a caustic alkali such as KOH at 800xc2x0 C. to 900xc2x0 C., removing the remaining alkali with heated water vapor, and using the obtained carbon. Furthermore, we have discussed the large capacitance generation mechanism of the electric double-layer capacitor using this carbon. Additionally, we have found various factors such as polarized electrodes used for the capacitor (i.e., the carbon electrodes) and solvents to impart higher energy density (given in Wh/Kg) and higher power density (given in W/Kg) to the electric double-layer capacitor. In this way, the present invention has been completed.
That is, the electric double-layer capacitor in accordance with the present invention has an organic electrolyte in which polarized electrodes are immersed. A carbon is used as a substance for activating the polarized electrodes and contains graphite-like crystallites of carbon. This carbon is a nonporous carbon having an interplanar spacing d002 of greater than 0.360 nm. Unlike the prior art activated carbon, at the beginning of assembly of a capacitor, the nonporous carbon has substantially no interface forming an electric double layer. When the applied voltage exceeds a certain threshold voltage during initial charging, ions of the electrolyte intrude into the carbon structure, together with the solvent. This phenomenon is known as solvent co-intercalation. This is the first time that an interface creating an electric double layer is formed. Subsequently, this interface is maintained by the hysteresis effect. The electric double-layer capacitor functions well.
Where an electrolyte solution including a solvent having a small molecular volume such as acetonitrile is used at this time, the solvent acts as a carrier having a high mobility into the carbon structure. Hence, the internal resistance can be reduced further. In addition, the power density can be enhanced further.
The xe2x80x9cnonporous carbonxe2x80x9d referred to herein is a carbon that can be obtained by the aforementioned method, for example. That is, the carbon does not have pores in sizes capable of accepting various kinds of electrolyte ions, solvents, N2 gas, and so on. The specific surface area measured by the BET method is less than 270 m2/g, more preferably less than 100 m2/g. Preferably, the interplanar spacing d002 of the carbon crystallites is in the range of 0.360 to 0.380 nm.
The nonporous carbon fabricated as described above has a hetero element-containing functional group such as COOH or CHO on its surface. The functional group is produced on the surface after an alkali treatment. This hetero element-containing functional group is removed by a thermal treatment at 500xc2x0 C. to 900xc2x0 C. within a reducing (redox) atmosphere such as a stream of H2 or a mixture gas of H2 and N2 obtained by decomposing NH3. In this way, the characteristics of the electric double-layer capacitor can be improved further.
The removal of the hetero element-containing functional group within carbon can be confirmed from the amount of hydrogen directly bonded to the carbon skeleton, the amount of hydrogen existing as chemically bonded, adsorbed water, and the amount of hydrogen existing as physically adsorbed water. These kinds of hydrogen in different states are present within the carbon. The amount of hydrogen directly bonded to the carbon skeleton appears as a short relaxation time component T2=18 to 50 xcexcsec (Gaussian type) during observation of 1H resonance of powdered carbon by pulse NMR as described later. The aforementioned chemically bonded, adsorbed water appears as a moderate relaxation time component T2=100 to 400 xcexcsec (Lorentzian type). The last-mentioned physically bonded, adsorbed water appears as a long relaxation time component T2=500 to 2000 xcexcsec or longer (Lorentzian type).
The intercalation mechanism of the electrolyte ions into the carbon structure as described above is similar to the mechanism of a carbon mesh plane adopted as the negative electrode of a lithium ion secondary battery. This carbon mesh plane has grown to some extent but its interplanar spacing is greater than that of graphite (d002 is greater than 0.337 nm) (for example, Japanese patent laid-open No. 145009/1999). This carbon barely reacts with the electrolyte solution, and lithium is occluded between the layers of the carbon. The present invention differs from the case of lithium ions in the following respects. The carbon has a greater interplanar spacing. Electrolyte ions are intercalated together with the solvent. This solvent exists between the layers without being decomposed unlike lithium ions. In addition, the solvent acts as a medium when ions go in and out. The intercalation increases the interplanar spacing. When a voltage is applied, the volume increases conspicuously. Moreover, the internal resistance is noticeably lower.
The nonporous carbon in accordance with the present invention increases in volume when a voltage is applied as described above. However, if a pressure that resists the pressure produced by the expansion is applied from the outside, and if the volume increase of the electrodes is suppressed completely, the capacitance produced between the electrodes remains the same as the case where free expansion is permitted. This fact reveals that the electric field application strongly urges the electrolyte ions to be intercalated between the layers together with the solvent.
If the distance between the current collectors is fixed during assembly of the electric double-layer capacitor, it follows that a pressure is applied to the current collectors because of expansion of the volume of the nonporous carbon, which in turn is caused by application of a voltage. This pressure is referred to as xe2x80x9cexpansion pressurexe2x80x9d. This expansion pressure can be checked by mechanically holding the outside of each electrode such that the length of the assembled capacitor taken in the direction of the electric field is kept constant and measuring the produced pressure with a pressure sensor such as a strain gauge. It is observed that a positive correlation exists between the measured expansion pressure and the capacitance of the electric double-layer capacitor. Preferably, the capacitor produces an expansion pressure exceeding 2 kg/cm2.
As described above, the electric double-layer capacitor in accordance with the present invention produces a capacitance by a mechanism different from the mechanism of an electric double-layer capacitor using the prior art activated carbon.
In particular, the electric double-layer capacitor in accordance with the present invention is an organic solvent-based electric double-layer capacitor using an organic electrolyte solution in which polarized electrodes are immersed. The polarized electrodes have crystallites of graphite-like carbon, and are made of a nonporous carbon having a specific surface area of less than 100 m2/g. The interplanar spacing d002 of the crystallites of the carbon is 0.360 to 0.380 nm.
From the nonporous carbon used in the electric double-layer capacitor in accordance with the present invention, hydrogen and oxygen atoms are removed except for hydrogen atoms directly bonded to the carbon skeleton. The reactive portions which would normally react with oxygen and water in air are terminated or blocked off by being replaced by hydrogen atoms. The difference between the bond states of hydrogen atoms remaining in the carbon structure can be found from the three relaxation time constants. That is, the short relaxation time component T2=20 to 50 xcexcsec (Gaussian type) is observed at 1H resonance by means of pulse NMR. The moderate relaxation time component is T2=100 to 400 xcexcsec (Lorentzian type). The long relaxation time component is T2=500 to 2000 xcexcsec or longer (Lorentzian type). This carbon can be confirmed when the ratio of the sum of the moderate and long relaxation times to the short relaxation time is less than one third. The capacitor is characterized in that the polarized electrodes are made of the carbon having these characteristics.
Where such relaxation times including the short, moderate, and long relaxation times T2=20to 50 xcexcsec (Gaussian type), T2=100 to 400 xcexcsec (Lorentzian type), and T2=500 to 2000 xcexcsec or longer (Lorentzian type), respectively, are observed at 1H resonance at room temperature by pulse NMR, if the ratio of the sum of the moderate and long relaxation times to the short relaxation time is less than one third, and if this carbon is used in an electric double-layer capacitor, it means that the selected carbon produces no gas during discharge, suffers from no attenuation of capacitance, and exhibits no increase in internal resistance. That is, this relation can be used as an index in selecting such an appropriate carbon. This index can be used in manufacturing an electric double-layer capacitor employing the prior art activated carbon, as well as in fabricating an electric double-layer capacitor using the nonporous carbon in accordance with the invention.
It is known, however, that it is essential to remove the hydrogen of the physically adsorbed water appearing as a relaxation time component T2=500 xcexcsec to milliseconds (Lorentzian type) (normally, 600 to 2000 xcexcsec), in order to maintain the characteristics of the organic solvent-type electric double-layer capacitor. In the past, such physically adsorbed water has been removed from the electrodes during assembly of the electric double-layer capacitor by vacuum degassing or other method. Therefore, these long relaxation time components do not normally exist. A carbon material that is used in electrodes in practice is required to satisfy the relation that the ratio of the moderate relaxation time component to the short relaxation time constant is less than one third.
Each polarized electrode of the electric double-layer capacitor in accordance with the present invention is fabricated by kneading together a nonporous carbon, a conduction-assisting agent, and a bonding agent and rolling the mixture into a sheet. Alternatively, after the kneading, the kneaded mixture is applied to a current collector. The polarized electrode is characterized in that the density of the obtained carbon electrode per apparent volume of the carbon electrode is 0.8 to 1.3 g/cm3. The density is measured from the weight when the electrode is in a dry state.
The electrolyte solution of the electric double-layer capacitor in accordance with the present invention is an aprotic solvent solution containing more than 0.5 mol/liter of an electrolyte selected from the group consisting of boric tetrafluoride salts of alkyl quaternary ammonium, phosphate hexafluoride salts, and perchlorate salts. The solvent has been prepared by selecting at least one from propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dimethoxyethane (DME), diethoxyethane (DEE), and xcex3-butyl lactone (xcex3-BL) as a solvent or a mixture solvent and compounding at least 30% molar fraction of acetonitrile (ACN) or a solvent having a molecular volume of less than 71 and a relative dielectric constant of more than 20.
That is, the selection of the electrolyte and the solvent forming the electrolyte solution depends on the intercalation mechanism in accordance with the present invention. Namely, the electrolyte ions are intercalated between the layers of the carbon together with the solvent. By selecting the electrolyte and solvent, the response speed can be increased, and the internal resistance can be decreased. As a result, an electric double-layer capacitor having an improved power density can be obtained.
Examples of alkyl quaternary ammonium electrolytes include Me4N+, EtnMe4-nN+, Et4N+, and (n-Bu)4N+. Examples of solvent having a molar molecular volume of less than 71 and a relative dielectric constant of more than 20 include propionitrile, ethylene carbonate, dimethyl sulfoxide, and nitromethane, as well as acetonitrile. Of these solvents, acetonitrile and propionitrile are preferred in that they have low viscosities, high decomposition voltages, and nearly equal DN value (solvent parameter electron donability) and AN value (solvent parameter electron acceptability).
The carbon electrode for use in the electric double-layer capacitor in accordance with the present invention increases in volume when a voltage is applied. Where the volume increases, even if the capacitance of the capacitor increases, the capacitance per unit volume is canceled out. Accordingly, the electric double-layer capacitor in accordance with the present invention is characterized in that it is equipped with a volume-suppressing means for suppressing increase in volume in the direction of the electric field. Since the expansion pressure is mainly produced in the direction of application of the voltage, it is not necessary to provide any volume-limiting means acting in other than the direction of application of the voltage. Where a sheet of electrode is rolled and received in a cylindrical container as the volume-suppressing means for resisting the expansion pressure, for example, the volume can be suppressed by fabricating the cylindrical container from a polymer film such as fluorocarbon or polyimide having a high tensile strength, since the expansion pressure is directed toward the outer periphery from the center. In the case of a laminated planar electrode, the volume suppression can be accomplished by squeezing a sheet of electrode between highly rigid pressure plates and insert the sheet of electrode in a bag-like sheet of a polymer such as fluorocarbon, polyimide, or polyamide having a high tensile strength in the same way.
The present invention also embraces a nonporous carbon for use in the electric double-layer capacitor and a method of fabricating the nonporous carbon.
A method of producing a graphite-like nonporous carbon having crystallites of carbon, a specific surface area of less than 270 m2/g (more preferably, less than 100 m2/g), and a crystallite interplanar spacing d002 of 0.360 to 0.380 um starts with dry distilling an easily graphitizable carbon having developed layers of crystallites of graphite at 700xc2x0 C.-850xc2x0 C. to obtain calcined carbon. The obtained calcined carbon is treated with a caustic alkali such as KOH, CsOH, or RbOH at 800xc2x0 C. to 900xc2x0 C. The remaining alkali is removed, for example, by water vapor under pressure. As an additional step, the obtained nonporous carbon is treated at 500xc2x0 C.-900xc2x0 C. within a reducing atmosphere (e.g., within a stream of hydrogen or within a mixture gas of 3H2+N2 obtained by decomposing NH3). Hydrogen and oxygen atoms excluding hydrogen atoms directly bonded to the carbon skeleton are removed. Also, radical electrons or trapped unpaired electrons, or reactive sites, that tend to react with oxygen and water within air are replaced by hydrogen atoms. In this way, hydrogenated nonporous carbon that is terminated or blocked off can be produced.
Strong alkalis such as LiOH, NaOH, KOH, CsOH, and RbOH can be used as the caustic alkali in the step described above. Where the sizes of ions are taken into consideration, KOH, CsOH, and RbOH are preferred among them. Furthermore, KOH is preferable in that it forms a charge transfer complex for promoting intercalation. In addition, KOH is preferable also from an economical point of view.
A carbon material in accordance with the present invention is a nonporous carbon for use in an electric double-layer capacitor and has crystallites of a graphite-like carbon and a specific surface area of less than 100 m2/g. The interplanar spacing d002 Of the crystallites of the carbon is 0.360 to 0.380 nm. The hydrogen remaining in the carbon structure that is measured with 1H resonance by pulse NMR is characterized in that the ratio of the sum of middle and long relaxation time components to the short relaxation time component is less than one third. The short relaxation time component is given by T2=18 to 50 xcexcsec (Gaussian type). The moderate relaxation time component is given by T2=100 to 400 xcexcsec (Lorentzian type). The long relaxation time constant is given by T2=500 to 2000 xcexcsec (Lorentzian type). The present invention is hereinafter described in further detail.